Multiple-Input Multiple-Output (MIMO) technology has increased the throughput of wireless systems due to the added Degrees of Freedom (DoF) which can be achieved by using spatial interference cancellation techniques. The sum capacity of a broadcast channel is achieved by Dirty Paper Coding (DPC). But due to the complexity and difficulty in implementing DPC in practice, several techniques based on the concept of precoding have been proposed offering different trade-offs between complexity and performance.
Block Diagonalization (BD) is an algorithm widely used to mitigate interference in wireless systems. It can be thought of as a generalization of channel inversion for scenarios that consider multiple antennas at reception. BD is one approach for linear precoding in the MIMO broadcast channel that sends multiple interference-free data streams to different users.
This algorithm can be implemented in several cellular scenarios. An example of a scenario is a single cell approach, which is composed of only one base station, equipped with an array of antennas, serving multiple data streams to users in a cell trying to avoid inter-user interference. Much analysis has been done in this scenario, and studies show that in some cases, where BD is combined with user selection algorithms, BD achieves a performance close to DPC.
Another interesting, and also promising, approach is to use BD in a Coordinated Multi-Point (CoMP) scenario, by jointly transmitting signals from multiple base stations. This method consists of grouping some geographically separated base stations that jointly process the signals of the users forming a virtual antenna array. This makes a CoMP transmission, to some extent, similar to the single base station scenario, with the difference being that now the users are located in different cells and the interference is referred to as Inter-cell interference (ICI). Nevertheless, this implies several practical issues on the transmit side, such as the need for a very fast backhaul connecting the base stations and per-base power constraints. This scenario is already considered in LTE-based networks and has been widely studied and assessed in different ways, through simulations and also by field trials.
Although CoMP with joint transmission is able to mitigate interference between multiple users within the CoMP cell, these users can still suffer interference from other systems or CoMP cells. Since this interference is caused by external nodes and cannot be coordinated, it is commonly referred to as uncoordinated or external interference. In new cellular networks, external interference may reach severe levels due to the use of smaller cells. Also, they can degrade the system performance as much as the internal interference.